build: standardize dependabot config and enable auto-merge for minor/patch updates

Groups minor/patch updates for auto-merge once CI passes, keeps majors as
individual PRs for manual review, adds a cooldown window, standardizes the
commit-message prefix to build(deps), and restores default dependency labeling.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.github/dependabot.yml

@@ -4,8 +4,15 @@ updates:
     directory: '/'
     schedule:
       interval: 'weekly'
+    cooldown:
+      default-days: 5
+    commit-message:
+      prefix: 'build'
+      include: 'scope'
     groups:
       dependencies:
         patterns:
           - '*'
-    labels: []
+        update-types:
+          - 'minor'
+          - 'patch'

.github/workflows/dependabot-auto-merge.yml

@@ -0,0 +1,25 @@
+name: Dependabot auto-merge
+
+on: pull_request
+
+permissions:
+  contents: write
+  pull-requests: write
+
+jobs:
+  auto-merge:
+    if: github.actor == 'dependabot[bot]'
+    runs-on: ubuntu-latest
+    steps:
+      - name: Dependabot metadata
+        id: metadata
+        uses: dependabot/fetch-metadata@25dd0e34f4fe68f24cc83900b1fe3fe149efef98 # v3.1.0
+        with:
+          github-token: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Enable auto-merge for minor/patch updates
+        if: steps.metadata.outputs.update-type != 'version-update:semver-major'
+        run: gh pr merge --auto --squash "$PR_URL"
+        env:
+          PR_URL: ${{ github.event.pull_request.html_url }}
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}